Lab 2 - Stratigraphy

Lab 2 - Stratigraphy - LAB # 2 - STRATIGRAPHY Stratigraphy...

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LAB # 2 - STRATIGRAPHY Stratigraphy is the study of rock layers, termed strata . Most stratified rocks are sedimentary, although extrusive igneous, metamorphosed sedimentary and metamorphosed volcanic rocks also commonly exhibit stratification. Stratigraphy involves three branches of study; lithostratigraphy, biostratigraphy and chronostratigraphy. Lithostratigraphy Lithostratigraphy defines rock units on the basis of their physical, or lithologic , features. Nicolas Steno, a Danish physician and scientist (1638-1686), founded the basic principles of stratigraphy including the principle of superposition, the principle of original horizontality and the principle of original lateral continuity. The Principle of Superposition states that in a sequence of undisturbed strata the oldest layer is on the bottom, the youngest is on top. Note that this assumes that the strata have not been overturned. If the strata have been deformed, there may be recognizable sedimentary features that indicate the original position of the strata. The Principle of Original Horizontality states that sedimentary layers are originally deposited horizontally and parallel to the surface. Therefore, if sedimentary layers are in a non-horizontal position they have been deformed. The Principle of Original Lateral Continuity observes that strata extend in all directions and either thin or abruptly end at the edge of a sedimentary basin. Thus, if portions of the originally continuous strata are eroded and no longer form a continuous unit, they may still be correlated. Correlation is the connecting together of equivalent rock units. This may be through comparison of lithologies (rock types), fossils, or facies relationships (rocks deposited at the same time in adjacent depositional environments). Charles Lyell, an English geology professor (1797-1875), further refined and defined several stratigraphic principles. These include the principle of cross-cutting relationships and the principle of inclusions. The Principle of Cross-cutting Relationships states that a geologic feature that cuts across another rock is younger than the rock it cuts across. Cross-cutting features often include igneous dikes, sedimentary structures and faults. Lyell also observed that fragments within larger rock masses are older than the rock masses in which they are enclosed. This is termed the Principle of Inclusions . Lithostratigraphic units are bodies of rock that are distinguished and defined by their lithologic (rock) characteristics and their stratigraphic position relative to other bodies of rock. They are usually layered sedimentary rocks that are separated from other lithostratigraphic units by contacts . Lithostratigraphic units include both formal and informal units. Formal rock unit names are always capitalized, and are named with an appropriate local geographic term combined with either a unit-term indicating its rank (Ex. = Group, Formation, Member) or a simple field lithologic term indicating the
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Lab 2 - Stratigraphy - LAB # 2 - STRATIGRAPHY Stratigraphy...

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